This invention pertains to ultrasound welding and concerns improvements made in "compound" sonotrodes in order to perform ultrasonic welding between semi-rigid polystyrene and molded cellulose. This invention applies especially to the closing of cellulose trays holding 20, 24 and 30 eggs under transparent polystyrene lids.
In the egg industry, trays of 20, 24 and 30 eggs under a transparent lid are wrapped in a retractable film.